This invention relates to solenoids in which the plunger is held or "latched" in position after deenergization by means of a permanent magnet.
Some applications for solenoids require that a solenoid plunger be moved to one position by instantaneous energization of a coil and held there indefinitely after the coil is deenergized. This holding or latching has been accomplished by a permanent magnet which holds until its flux is countered by opposing flux from the coil.
Other applications require that the plunger be held in either end position when no power is applied to the solenoid. This has required two separate permanent magnets, one at each end of the solenoid.
In U.S. Pat. No. 4,514,710, Apr. 30, 1985, this double holding action is achieved with a single permanent magnet mounted on the solenoid frame midway between its two ends. This establishes a magnetic flux around both ends of the frame and through the plunger. This flux holds the plunger in the end it happens to be in.
The patented construction, however, requires a costly large ring of magnetic material around the plunger tube for distributing magnetic flux to the plunger. It also involves assembly steps making the double latching solenoid quite expensive.